1. Field of the Invention
The present invention concerns flexure-type suspension systems, flexure-type positioning assemblies based thereon and flexure-type fine positioning assemblies for the positioning of optical elements. Precision tip-tilt piston actuators providing exact constraint are addressed in particular.
A suspension system has to comply with a number of requirements, depending on the field of use. Typically, a suspension system serves as a mechanical interface between two points A and B, whereby the point B can move or can be moved with respect to the point A. A suspension typically restricts some of the degrees of freedom. A suspension may have a (linear) stiffness in Rx, Ry and Tz directions, for instance.
Suspension systems for suspending optical elements are for example employed to protect a movable mirror or lens against vibrations and to allow the mirror or lens to be tilted.
There are positioning assemblies that comprise a suspension system. In particular for positioning assemblies it is preferable to guarantee a linear stiffness.
Fine positioning assemblies are usually employed in order to provide for an accurate alignment of two remote systems. Fine positioning assemblies are for example used in optical systems where a laser transmitter (source) is to be aligned with respect to a receiver (target). Dynamic fine positioning assemblies are of particular importance, since many optical systems require a continuous adjustment between the source and the target. In the past, dynamic fine positioning assemblies were mainly used in a research environment since they were bulky and expensive. With the improvement of the positioning assemblies and with the reduction of their manufacturing cost, they are now more widely used in communication systems and measurement systems, for instance.
2. Related Prior Art
A known suspension system with a membrane suspension is schematically depicted in FIG. 1A. A membrane 1 is employed to suspend a mirror 4. The membrane 1 is preferably parallel to the mirror surface. The membrane can be made to have cut outs. Usually, the stiffness in Tz and strength in Tx and Ty is compromised for compliance in Rx and Ry. An example of a membrane-type fine positioning assembly similar to the one depicted in FIG. 1A is described in the paper “A Fine Pointing Mechanism For Intersatellite Laser Communication”, P. Bandera, Proceedings of the 8th European Space Mechanisms and Tribology Symposium, Toulouse, France, 29 Sep.—1 Oct. 1999, ESA SP-438. The assembly comprises a BeCu diaphragm (membrane) serving as suspension for a mirror. The periphery of the diaphragm is fixed by means of screws to a mirror assembly. It is a disadvantage of this design that it is critical to strain (e.g., thermal strain). Under certain circumstances, the diaphragm shows a non-linear behavior and cannot be made to retain its position and stiffness characteristics. It is a further disadvantage that the stiffness coefficients Tz, Rx, and Ry change over the working range. There are also problems with the mechanical integrity of the suspension system reported. Mild mechanical shocks can lead to large unpredictable changes in mechanical behavior.
In FIG. 1B a known approach is illustrated. This approach is based on the approach depicted in FIG. 1A. In addition to the membrane 1 a post 2 is provided underneath the membrane 1. The post 2 gives vertical stiffness in Tz. The post 2 unfortunately adds to the stiffness in Rx and Ry for the same strength in Tx and Ty. An example of a fine positioning assembly in accordance with FIG. 1B is described in the patent U.S. Pat. No. 5,110,195 assigned to the Massachusetts Institute of Technology (MIT). The assembly comprises a flexure ring with spokes serving as suspension for a mirror. In other words, a membrane with cut outs is employed. This design shows about the same disadvantages as the design by P. Bandera described above.
A suspension system with wire suspension is shown in FIG. 1C. An example of suspension system with wire suspension is described and claimed in the patent U.S. Pat. No. 4,973,145 assigned to Lockheed Missiles & Space Company. The assembly comprises a plurality of flexible support elements 3 (e.g., wires) being at their lower end connected to a foundation plate and at their upper end via a ring-shaped element—referred to as mounting plate—to a mirror 4. This design is complex and the assembling of this assembly is assumed to be cumbersome since the joining of the wires to the mirror 4 is difficult.
A cardanic flexure suspension is depicted in FIG. 1D. The flexure system consists of two perpendicular four-bar linkages 5.3, 5.4 coupled to a common plate 5.1. The upper ends of the linkages 5.3 are attached to ground and the upper end of the linkages 5.4 are fixed to a mirror frame 5.2 carrying the mirror 4. One example of a cardanic flexure-type fine positioning assembly having only two degrees of rotational freedom is described in the patent U.S. Pat. No. 5,529,277. This patent is assigned to Ball Aerospace, USA. There are various papers that address the Ball Aerospace flexure suspension and similar designs. This fine positioning assembly uses two perpendicular four-bar linkages coupled by a common part to provide a suspension for an object to be supported. It is a disadvantage of this assembly that the inertia of the interface can not be made uniform in all tip tilt directions. Comparable approaches are addressed in the German patent DE-19700580 and in the European Patent EP-0 665 389 currently assigned to Carl Zeiss, Germany.
Another type of suspension is called cardanic element suspension. It comprises a monolithic central hinge flexure. These kind of systems tend to have a low life expectancy when being exposed to high loads e.g., during the launch phase of a rocket.
Universal pivot suspensions are depicted in FIGS. 1E and 1F. Both suspension system comprise a post 7 suspending the mirror 4 in a single point provided inside a cone shaped support element 8. According to FIG. 1E, the mirror 4 is pulled onto the post 7 by means of two or more springs 6. Likewise, permanent magnets 9.1 and 9.2, as illustrated in FIG. 1F, can be used to pull the mirror 4 towards the post 7. The dynamic and static friction is small. Typically a diamond/diamond combination is used for the post 7 and cone 8. There are a number of patents that seek to apply this approach. A non-exhaustive listing is: U.S. Pat. No. 3,946,166, U.S. Pat. No. 4,100,576, U.S. Pat. No. 4,175,832, U.S. Pat. No. 4,073,467, and U.S. Pat. No. 4,157,861.
In addition to those approaches that are specifically mentioned, there are many other approaches known that typically are combinations or modifications of the six different approaches given in FIGS. 1A through 1F.
Conventional fine positioning assemblies which use membranes or membrane-like members are known to show a so-called “clicking effect”, where the membrane from time to time jumps from one position to another position. This non-linear effect is a great disadvantage of known assemblies since it may lead to situations where the position of the membrane and thus the position of an optical element carried by the membrane is not defined anymore. Due to this non-linear behavior, the movements of the fine positioning assembly are not fully predictable.